This invention relates to high pressure sodium lamps and in particular to circuit arrangements which provide high voltage of pulses for initiating conduction of such lamps.
High pressure sodium discharge lamps generally require a high voltage pulse to initiate conduction. Such a high voltage pulse can be generated, for example, by using a ballast with two windings and using an SCR to discharge a capacitor through one of the windings at a time when there is sufficient voltage across the lamp to sustain conduction.
The commonly used prior art circuit utilized a glow lamp circuit such as shown in FIG. 1 to initiate conduction. In such a circuit the timing for firing the SCR is developed from the voltage across C1 (the capacitor used to store energy) by using R3 and C2 to give a phase shifted (delayed) voltage to G1. When a high enough voltage appears across the glow lamp G1, it breaks down and triggers the SCR. Thus the SCR is triggered by a voltage which is not in phase with the voltage across C1.
Although this prior art circuit functioned satisfactory in many applications, some problems were encountered. First, difficulties were encountered in fabrication of the circuit due to the relatively wide manufacturing tolerance in "break down voltage" of a given type of glow lamp. Secondly, it was found that the starting circuits often did not function at slightly lower than normal line voltages.